1. Field of the Invention
The present invention relates to a composite material composed of fibrous boride of at least one element belonging to Groups IVa to VIa of the Periodic Table and silicon carbide, said composite material being suitable for use as machine parts. The present invention also relates to a process for producing said composite material.
2. Description of the Related Art
Attempts have been made to apply silicon carbide to high temperature mechanical components because it exhibits outstanding mechanical and chemical properties at high temperatures. Silicon carbide retains its strength, or even increases in strength above that it has at room temperature, at high temperatures above 1500.degree. C. when it is incorporated with a sintering additive based on boron or a boron compound as disclosed in Japanese Patent Publication Nos. 32035/1982 and 34147/1984. Therefore, it is considered to be a promising material for gas turbine engines which operate at high temperatures above 1400.degree. C.
Despite the sintering additive, silicon carbide still suffers from a serious disadvantage of having a low value of fracture toughness (K.sub.IC value). Boron-containing silicon carbide formed by hot pressing and boron- and carbon-containing silicon carbide formed by pressureless sintering have a K.sub.IC value of 2.7-2.8 MPa.multidot.m.sup.1/2 when measured by the IM (indentation microfracture) method. This value is about one half the K.sub.IC value (5-7 MPa.multidot.m.sup.1/2) of Si.sub.3 N.sub.4 sintered product. This stems from the fact that the fracture of boron-containing silicon carbide is transgranular fracture which produces a smaller surface area of fracture and hence consumes a smaller amount of fracture energy.
In the case of SiC-based material in which Al.sub.2 O.sub.3 is used as a sintering additive, fracture takes place mainly along grain boundaries. In some cases, it has a K.sub.IC value in excess of 6 MPa.multidot.m.sup.1/2 (as reported by Suzuki at the Fourth Koonzairyo-Kiso-Toronkai, 1984, pp. 31-34). Nevertheless, Al.sub.2 O.sub.3 -containing silicon carbide is not used for gas turbine engines because it decreases in strength at high temperature above 1200.degree. C.
Several attempts have been made to improve the fracture toughness of SiC-based materials. They include the dispersion of particles of a particular substance in a SiC-based material. For example, particles of titanium diboride (TiB.sub.2) are dispersed in a SiC-based material. (See Japanese Patent Laid-open No. 27975/1982, and Am. Ceram. Soc. Bull., vol. 66, No. 2, 1987 p. 322-324 and p. 325-329.) The SiC-based material containing TiB.sub.3 particles exhibits a high value of fracture toughness. Such a SiC-based material can be produced by mixing SiC powder, TiB.sub.2 powder, and sintering additives, followed by molding and sintering. It can also be made by adding TiB.sub.2 powder to SiC powder and then adding B and C, and hot pressing the resulting mixture.
The dispersion of zirconium diboride (ZrB.sub.2) in place of TiB.sub.2 was also reported (in Yogyo Kyokaishi, vol. 93, No. 3, 1985, p. 123-129).
The borides reported so far are limited in their contribution to the improvement of fracture toughness because they are particles of equiaxial shape.
It is theoretically known that a composite material has an extremely high K.sub.IC value if it is incorporated with particles having a high aspect ratio like short fibers. (See K. T. Faber and A. G. Evans, Acta Metall., vol. 31, No. 4, p. 565-576.) It has also been experimentally proved that alumina incorporated with SiC whiskers has a higher K.sub.IC value than alumina incorporated with SiC particles. (See Tani and Wada, Yogyo Kyokai Tokai Shibu Gakujutsu Kenkyu Happyokai Koen Yoshishu, 1986, p. 21.)
So far, nothing has been known about short fibers of metal borides, although long fibers of TiB.sub.2 have been produced by the CVD method on a trial basis. Inorganic fibers produced by the CVD method are expensive and not suitable for ceramic composite materials produced by the ordinary powder process because they are generally thicker than 100 .mu.m in diameter. In addition, metal borides in fibrous form would be difficult to handle because they have a high specific surface area and hence to readily react with water.